Certain characteristics of liquids have traditionally been measured in order to develop a composite of the overall quality of the liquid. Liquid characteristics that have been measured to develop such a composite include the following: pH, turbidity, amount of dissolved oxygen, suspended solids, and dissolved solids. Devices used for performing these measurements (specific examples being the pH meter and dissolved oxygen and turbidity probes) typically require special handling, frequent adjustment, and manual calibration.
Sensor probes for remotely monitoring impurity levels in a liquid, such as water, are typically based on pH sensors, which require special handling and periodic calibration, thus limiting their use in remote applications. Other sensors for water, such as dissolved oxygen and turbidity probes, require even more frequent calibration. Water sampling kits, such as the Hach Kit, are inexpensive; however, the need for manual intervention during tests using such equipment is high, therefore rendering the process costly and inconvenient. Also, known sensors for monitoring liquids in process control systems in manufacturing must be vigilantly maintained, adjusted, and manually calibrated by specially trained technicians. Because of the harsh environments within which some of the known sensors must be capable of operation, they are expensive to maintain, as well as to buy.
U.S. Pat. No. 5,473,934, "Ultrasonic Fluid Composition Monitor," issued to Wesley Cobb on Dec. 12, 1995, describes a method of, and apparatus for, continuously monitoring the composition of a fluid mixture traveling through a conduit, where the fluid mixture can be a liquid/liquid or a liquid/solid. Ultrasonic propagation parameters and temperature are measured and compared to calibrated data based on analytical measurements of samples of the process fluid mixtures. Since this monitor is directed only to analyzing a flowing sample with equipment clamped to the outside of a conduit; it is not conducive to field (remote) use.
U.S. Pat. No. 5,060,507, "Method and Apparatus for Fluid Mixture Monitoring, Constituent Analysis, and Composition Control," issued to John Urmson, et al, on Jun. 21, 1989, describes another method and apparatus for fluid mixture monitoring and controlling using an acoustic sensing technique. A fluid sample and a reference fluid in elongated chambers are pulsed by sound waves; the system senses either the resonant frequencies in the chambers or the time lapse for the sound waves to traverse the chambers and determines composition of the fluid mixture by ratiometrically comparing the time-based measurements. The complexity of the device and the nature of the interaction between the reference fluids indicate that this monitor is also not conducive to field work.
U.S. Pat. No. 4,630,482, "Method and Apparatus for Ultrasonic Measurements of a Medium," issued to John Traina on Jun. 17, 1985, describes yet another method and apparatus for measuring the time required for an ultrasonic tone burst to traverse a medium from a transmitter to a receiver. Two transducers are required; a demodulator for high turbulence environments is preferred.
There is a need for a simple sensor system designed to speedily and easily identify chosen characteristics in liquids that avoids the problems typical of known monitors, i.e., complex equipment, elaborate electronics, complex sampling techniques, continual maintenance, frequent adjustment, and manual adjustment. There is also a need for a low-maintenance sensor system suitable for remote operation, i.e., that can be used without human operators and/or in situ. There is, still further, an ongoing need for miniaturized, easily handled sensor systems.